1. Field of the Invention
This invention relates to pipeline systems and more particularly to an assembly for receiving and bypassing pipeline "pigs" inserted into the pipeline system. Pipeline pigs are used for cleaning the interior of a pipe, separating different materials being pumped through the pipe, or the like. More particularly, this invention relates to an assembly for bypassing pipeline pigs around a "slug" catching apparatus which receives liquid accumulations in a natural gas pipeline which are scavenged by the pipeline pig moving therethrough.
2. Description of the Prior Art
Recovery of offshore natural gas involves the use of an offshore drilling platform for the drilling of natural gas wells. After the well is drilled, a pipeline connects the well head to the platform. Compressors mounted on the platform compress the natural gas and deliver it via another pipeline along the ocean floor to a centrally located recovery platform. Some subsea wells are drilled from a drilling ship and no platform is used. A pipeline connects the wellhead along the sea floor directly to a recovery platform, the wellhead pressure being sufficient to convey the natural gas without the need for compressors.
Typically, a number of drilling platforms supply natural gas via individual pipelines to the recovery platform. The recovery platform again recompresses the natural gas and conveys it via another pipeline to shore-based handling facilities.
Natural gas exiting a wellhead typically has a temperature in excess of 100.degree. F. As the natural gas travels along the pipeline from the drilling platform to the recovery platform, the relatively colder sea water cools the natural gas. As the natural gas cools, various components condense into liquid form. As these condensed products accumulate in the pipeline, the flow of the vapor components of the natural gas is reduced.
When the flow of natural gas through the pipeline is reduced to an unacceptable level, it becomes necessary to remove the condensibles from the pipeline. This is accomplished by inserting a pipe pig into the drilling platform end of the pipeline. A pipe pig is typically a spherical, hollow, neoprene body filled with antifreeze and having a diameter approximately equal to the interior diameter of the pipeline. The pressure generated by the compressors on the drilling platform or the natural wellhead pressure of the gas moves the pig along the pipeline toward the recovery platform. The pig is resilient and flexible and the pressure behind the pig forces it into tight contact with the interior walls of the pipeline. As the pig moves along the pipeline it pushes the accumulated liquid or "slug" before it. A slug catcher on the recovery platform separates the accumulated liquid from the natural gas and stores the liquid. Other types of pipe pigs are sometimes used--for example, a bullet-shaped pig with concentric seal rings.
Typically, the quantity of accumulated liquid is so great that the slug catcher fills up before the pig has completely emptied the pipeline of liquid. This accumulation of liquid necessarily halts the transport of natural gas until the liquid can be processed and separately pumped to shore. The processing often takes 24 to 48 hours, during which time natural gas production is halted.
A recovery platform big enough to hold additional or larger slug catchers is prohibitively expensive. One prior art solution involves placement of a very large slug catcher on the ocean floor in the vicinity of the recovery platform. When an ocean floor based slug catcher is used, divers must be sent down to the ocean floor to recover the pig from the pipeline. As one skilled in the art will appreciate, this can be a dangerous, expensive, and time-consuming activity.